[CoCl(TPPyP)], a novel bifunctional catalyst for the coupling reaction of carbon dioxide and propylene oxide

Article abstract of DOI:10.3184/030823410X12762744367622

A novel bifunctional catalyst chloro[meso-tetrakis(1-phenylpyrazol-4-yl) porphyrin] cobalt(III), [CoCl(TPPyP)], has been prepared and used to catalyse the synthesis of propylene carbonate from carbon dioxide and propylene oxide under mild conditions. Propylene carbonate was obtained with a yield of 25.2% at 100 °C in the absence of co-catalyst. A reaction mechanism is proposed.

Full text of DOI:10.3184/030823410X12762744367622

JOURNAL OF CHEMICAL RESEARCH 2010
RESEARCH PAPER 361
JULY, 361–364
[CoCl(TPPyP)], a novel bifunctional catalyst for the coupling reaction of
carbon dioxide and propylene oxide
Tiegang Rena, Weijie Lia, Zhanwei Bua,b*, Lirong Yanga and Zhiqiang Wanga
aFine Chemistry and Engineering Research Institute, Henan University, Kaifeng 475004, Henan, P. R. China
bHenanTianguan Enterprise Group Co., Ltd., Nanyang 473000, Henan, P. R. China
A novel bifunctional catalyst chloro[meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin] cobalt(III), [CoCl(TPPyP)], has
been prepared and used to catalyse the synthesis of propylene carbonate from carbon dioxide and propylene
oxide under mild conditions. Propylene carbonate was obtained with a yield of 25.2% at 100 °C in the absence of
co-catalyst. A reaction mechanism is proposed.
Keywords: chloro[meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin]cobalt(III), carbon dioxide, propylene oxide, propylene
carbonate, bifunctional catalyst
The chemical fixation of carbon dioxide has been extensively
studied recently, due to the demand for a low-cost carbon
source and the urgent need to reduce the levels of greenhouse
gases.1–3 One of the most promising reactions applicable to the
chemical fixation of carbon dioxide is the synthesis of pro-
pylene carbonate from CO2 and propylene oxide (Scheme 1).
Since commercialisation in the mid-1950s, propylene carbon-
ate has found numerous applications as an attractive solvent, a
reactive intermediate, a fine chemical, etc.4,5
Various catalysts including metal oxides, zeolites, organic
bases, alkali metal halides, ionic liquids, quaternary ammo-
nium salts, salen complexes, phthalocyanine complexes
and metalloporphyrin complexes have been successively
developed for the coupling reaction of CO2 and propylene
oxide.6–16 In particular, metalloporphyrinates, as a novel class
of catalysts for preparing polycarbonates or carbonates from
carbon dioxide and epoxides, have been found to exhibit high
activity towards the coupling reaction. Inoue et al.17,18 found
that the copolymerised product of carbon dioxide and epoxides
in the presence of aluminum porphyrin as the catalyst had a
narrow distribution of molecular weight which could be well
controlled by adjusting reaction conditions. Kruper and Deller19
reported the catalyst system [CrCl(TPP)] /DMAP (dimethyl
aminopyridine) for preparing propylene carbonate from CO2
and propylene. Since then, various similar catalyst systems
such as [Co(TPP)X]/DMAP,20 Cu(II) porphyrin/DMAP,16
[M(TPP)X]/PTAT(M = Co, Mn, Fe, Ru; X = Cl, Br, OAc,
OTs)21 have been reported. Unfortunately, the above-
mentioned catalysts exhibited very low or even zero catalytic
reactivity for the coupling reaction of CO2 and propylene oxide
in the absence of co-catalysts (e.g., 1-methylimidazale, DMAP,
quaternary ammonium, phosphonium salts etc.). For example,
17
Inoue et al. found that 1-methylimidazale as a co-catalyst
played a key role in the coupling reaction of CO2 with pro-
pylene oxide generating propylene carbonate, which prompted
us to consider that N-phenylpyrazole might play the same role
as 1-methylimidazale in the reaction of CO2 and epoxides,
similar to that reported by Inoue et al. Therefore, we synthe-
sised the complex [CoCl(TPPyP)] (Scheme 2), in which one
Scheme 1 The synthesis of propylene carbonate (PC) from
CO2 and propylene oxide.
Scheme 2 Structure of chloro-[meso-tetrakis(1-phenylpyrazole-4-yl) porphyrin] cobalt (Ш).
* Correspondent. E-mail: buzhanwei@henu.edu.cn

362 JOURNAL OF CHEMICAL RESEARCH 2010
Coupling reactions of CO2 and propylene oxide(PO); general
procedures
N-phenylpyrazole ring is linked to the meso-position of por-
phyrin ring and the Co-porphyrin is the Lewis acidic centre
and phenylpyrazole is the Lewis base segment. This means
that [CoCl(TPPyP)] should have bifunctional catalytic activity
for the coupling reaction of CO2 with propylene oxide.
The reactions were carried out in a 100 mL stainless steel autoclave
equiped with a magnetic stirring device. In a typical procedure, the
autoclave reactor (dried in vacuo) was charged with [CoCl(TPPyP)]
(0.024 g, 0.025 mmol) and DMAP (0.006 g, 0.05 mmol) then purged
with nitrogen gas. Then propylene oxide (8.3 g, 0.143 mol) and
CH2Cl2 (1 mL) were added into the reactor with a hypodermic syringe,
followed by pressurising with CO2 to 4.5 MPa and stirring at the
desired temperature (80–120 °C). Upon completion of the reaction,
the reactor was cooled, and the product was transferred to a round-
bottom flask after discharging excessive CO2. Residual reactants and
solvent were removed in vacuo and product propylene carbonate was
isolated as a colourless liquid via Kugelrohl distillation. Characterisa-
tion of the product was by 1H NMR (CDCl3) δ: 1.50 (d, J = 6.24 Hz,
3H, CH3), 4.05 (t, J = 7.82 Hz, 1H, CH2CH), 4.59 (t, J = 8.02 Hz, 1H,
CH2CH), 4.82–4.92 (m, CH).
Experimental
Melting points were measured on Netzsch STA449C Simultaneous
Thermal Analysis spectrometer. Mass spectra were determined on an
Agilent 1100LC-MS mass spectrometer; 1H NMR spectra were
recorded on an INOVA-400 spectrometer in CDCl3 using TMS as an
internal standard. Elemental analyses were performed with a PE2400
elemental analysis apparatus. Tetramethoxypropane and phenylhy-
drazine were commercially available and used without further puri-
fication. Pyrrole was distilled under reduced pressure before use.
Commercial CO2 (99.99%) was used without further purification.
Propylene oxide (PO) was distilled from CaH2 under nitrogen before
use. CH2Cl2 was washed successively with concentrated H2SO4, water,
aqueous NaHCO3, and brine, dried over CaCl2, and distilled over
CaH2 under dinitrogen. DMAP were purchased from Aldrich and used
without further purification.
Results and discussion
Synthesis of the catalyst
Two established methods for porphyrin synthesis via the tetra-
cyclisation of aldehydes and pyrrole developed by Adler and Lindsey,
respectively, had been reported to yield pyrazole substituted por-
phyrins.22,26,27 With slight variations of the reaction conditions, we
found that it was possible to obtain meso-tetrakis(1-phenylpyrazol-
4-yl)porphyrin in improved yields compared to the Lindsey method.
For an abundant electron five-membered aromatic heterocycle, it is
more difficult for the tetra-cyclisation of 1-phenyl-4-formylpyrazole
with pyrrole than with benzaldehyde. The existence of nitrobenzene
can increase the reaction temperature, and as an oxidant it was
also advantageous to improve the yield. Furthermore, the workup
procedure was simplified allowing the synthesis and purification of
porphyrin in a scale of up to 40–50 mmol of starting material in two
days.
Synthesis of the catalyst
Meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin (TPPyPH2)
Meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin (TPPyPH2) was syn-
thesised according to the literature.22 A three-necked round-bottomed
flask fitted with a reflux condenser was charged with propionic
acid (70 mL) and nitrobenzene (30 mL). 1-Phenyl-4-formylpyrazole
(1.72 g, 10 mmol) and pyrrole (0.67 g, 10 mmol) were added at near
reflux, then the reaction was refluxed for 1 h. After the removal of the
solvent, the residue was purified by column chromatography on neu-
tral alumina eluting with CH2Cl2/Et2O. The first red fraction was col-
lected and further purified by column chromatography on silica gel
eluting with CH2Cl2/Et2O to offer the final product: 0.21 g, 13.6%.
Mp. 305.9 °C; MS (m/z) 879 and 880; UV(CHCl3): λmax(nm)/logε:
425(5.62), 523(4.33), 564(4.36), 660(4.41). 1H NMR (CDCl3) δ:
−2.591 (s, 2H, NH), 7.430–7.466 (t, 4H, Ph-H, J = 7.2 Hz), 7.617–
7.657 (t, 8H, Ph-H, J = 8.0 Hz), 8.089 (d, 8H, Ph-H, J = 8.0 Hz), 8.618
(s, 4H, pyrazole-H5), 8.80 (s, 4H, pyrazole-H3), 9.235 (s, 8H, pyrrole-
Hβ). Anal. Calcd for C56H38N12: C:76.52; H:4.36; N:19.12. Found:
C, 76.31; H, 4.62; N, 19.05%.
Coupling reaction of CO2 and propylene oxide(PO)
The presence of both a Lewis acid centre and a Lewis base centre is
essential for the high efficiency and selectivity of the catalyst. So we
designed a catalyst which owns both centres and its structure is shown
in Scheme 2. The cobalt-porphyrin acts as a Lewis acid centre and the
phenylpyrazole as another.
It is imperative to incorporate a Co–Cl bond into the synthesised
catalyst, otherwise no propylene carbonate is obtained from the cou-
pling reaction of CO2 with propylene oxide. Thus when [Co(TPPyP)]
untreated with concentrated hydrochloric acid was used as the catalyst
in the coupling reaction of CO2 with propylene oxide, no target prod-
uct was harvested (Table 1, entry 1). Similarly, even in the presence
of co-catalyst DMAP, [Co(TPPyP)] untreated with concentrated HCl
still had no catalytic activity for the coupling reaction of CO2 with
propylene oxide (Table 1, entry 2). However, after being treated with
concentrated HCl, the resulting [CoCl(TPPyP)] exhibited signifi-
cantly increased catalytic activity for the same coupling reaction
(Table 1, Entries 4–7 ). In a first attempt to catalyse the coupling reac-
tion of CO2 and propylene oxide, [CoCl(TPPyP)] was used as the
catalyst without co-catalyst. When the reaction was carried out at
100 °C for 24 h, a moderate PC yield of 25.2% with a TON of 1391
and a TOF for 58 h−1 were obtained (Table 1, entry 3). To the best
of our knowledge, all of the [M(TPP)X] catalysts suffer from no
catalytic reactivity or very low catalyst reactivity in absence of the
Meso-tetrakis(1-phenylpyrazol-4-yl)porphyrincobalt(II), [Co(TPPyP)]
TPPyPH2 (2.86 g, 3.25 mmol) was refluxed for 1 h in acetic acid with
cobalt chloride hexahydrate (1.5 equiv of TPPyPH2) and sodium ace-
tate.23–25 The crystalline product was collected by filtration and washed
with water, aqueous sodium bicarbonate, water, and methanol and was
dried in vacuo to give a red solid of [Co(TPPyP)]. Yield: 3.04 g, 91%.
UV (CH2Cl2 ), λmax/nm: 423, 534, 605. IR(cm−1): 1001.37 (Co-N).
Chloro[meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin]cobalt(III),
[CoCl(TPPyP)]24,25 To a suspension of [Co(TPPyP)] (1.0 g, 1 mmol)
in MeOH (500 mL) was added 3 mL of aqueous 37% HCl solution.
The mixture was stirred at room temperature in an open flask for 5 h
and then filtered to give a purple solid. Recrystallisation from CHCl3-
EtOH to gave the final product. Yield: 0.83 g, 85%. UV (CH2Cl2),
λmax/nm: 429, 550, 656. ESI-MS (m/z) Calcd (M-Cl)+ 935.9, found:
935.3. Anal. Calcd for C56H36N12ClCo: C, 69.24; H, 3.74; N, 17.30.
Found: C, 69.46; H, 3.87; N, 17.00%.
Table 1 Reaction of CO2 and propylene oxide catalyzed by [CoCl(TPPyP)]a
Entry
Catalyst/co-catalyst
Time/h
Temperature/°C
Yield/%b
TONc
TOFd
1
2
3
4
[Co(TPPyP)]
[Co(TPPyP)]/DMAP
[CoCl(TPPyP)]
24
24
24
24
100
100
100
100
No
No
25.2
23.7
–
–
–
–
58
54.4
1391
1304
[CoCl(TPPyP)]/DMAP
a Reaction conditions: [CoCl(TPPyP)] (0.025 mmol), DMAP (2 equiv), propylene oxide (10 mL, 5716 equiv), CO2(4.5MPa), CH2Cl2
(1mL).
b Isolated yield.
c Moles of propylene oxide produced per mole of cobalt complex.
d Moles of propylene oxide produced per mole of cobalt complex per hour.

JOURNAL OF CHEMICAL RESEARCH 2010 363
Table 2 The coupling reaction of CO2 and propylene oxide by the cobalt porphyrin systema
Entry
Catalyst/co-catalyst
Time/h
Temperature/°C
Yield/%b
TONc
TOFd
1
2
[CoCl(TPPyP)]
24
24
24
24
24
24
24
24
18
6
80
90
100
110
120
80
Trace
25
–
–
57.5
58
38.4
31.9
48.8
54.4
160.1
56.2
193.9
[CoCl(TPPyP)]
1379
1391
921
3
[CoCl(TPPyP)]
25.2
16.7
13.9
21.3
23.7
69.7
18.3
21.5
4
[CoCl(TPPyP)]
5
[CoCl(TPPyP)]
766
6
[CoCl(TPPyP)]/DMAP
[CoCl(TPPyP)]/DMAP
[CoCl(TPPyP)]/DMAP
[CoCl(TPPyP)]/DMAP
[CoCl(TPPyP)]/DMAP
1172
1304
3844
1012
1164
7
100
120
90
8
9
10
120
aReaction conditions: [CoCl(TPPyP)] (0.025 mmol), DMAP (2 equiv.), propylene oxide (10 mL, 5716 equiv.), CO2 (4.5 MPa), CH2Cl2
(1mL).
bIsolated yield.
cMoles of propylene oxide produced per mole of cobalt complex.
dMoles of propylene oxide produced per mole of cobalt complex per hour.
co-catalysts. The results proved our assumption that the phenylpyr-
azole worked as a Lewis base centre for this reaction.
propylene carbonate was obtained in a high yield of 69.7% after
24 h reaction at a relatively higher temperature of 120 °C (Table 2,
entry 8).
It is to be noted that at a reaction temperature of 100 °C, [CoCl
(TPPyP)] showed a higher reactivity for the coupling reaction than
[CoCl(TPPyP)]/DMAP (Table 2, entries 3 and 7). This implies that
the co-catalyst DMAP is sensitive to reaction temperature in terms of
its catalytic efficiency for the coupling reaction of CO2 with propylene
oxide.
Preliminary tuning of reaction conditions
The reaction conditions of the coupling reaction of CO2 and propylene
oxide have been studied tentatively. The results are listed in Table 2.
When propylene oxide (5716 equiv. to [CoCl(TPPyP)]) was added
in a stainlesss steel autoclave and pressurised by CO2 at 4.5 MPa and
90 °C in the presence of [CoCl(TPPyP)], the reaction of CO2 and pro-
pylene oxide proceeded smoothly, generating propylene carbonate at
a yield of 25% in 24 h (Table 2, entry 2). While the other conditions
were kept unchanged but temperature was increased to 100 °C, 110 °C
and 120 °C, propylene carbonate was obtained at a yield of 25.2%,
16.7%, and 13.9%, respectively (Table 2, entries 3, 4 and 5). As
an exception, trace propylene carbonate was obtained and 0.0589 g
of poly(propylene carbonate) were collected in the presence of
[CoCl(TPPyP)] at a lowered temperature of 80 °C (Table 2, Entry 1).
This indicates that temperature has a significant effect on the coupling
reaction.
Possible mechanism
It has been proposed that the coupling of carbon dioxide with epox-
ides to yield cyclic carbonates probably requires activation through
both a Lewis acid and a Lewis base: the former activates the epoxide
and the latter attacks the less sterically hindered carbon atom to open
the epoxide ring. The generated oxy-anion species then reacts with
CO2 to give the corresponding cyclic carbonate.28,29
Based on the molecular structure of [CoCl(TPPyP)] and coupling
reaction results, we propose a possible mechanism for the coupling
reaction of propylene oxide and carbon dioxide (Scheme 3) that is
similar to that reported by others.17,20 In this mechanism, as illustrated
in Scheme 3, the Lewis acid centre of [CoCl(TPPyP)] first coordinates
Moreover, when co-catalyst DMAP was introduced into the cou-
pling reaction system of CO2 and propylene oxide, increased
reactivity was generally obtained (Table 2, entries 6–8). In particular,
Scheme 3 Proposed mechanism for the coupling reaction of CO2 with propylene oxide catalyzed by [CoCl(TPPyP)].

364 JOURNAL OF CHEMICAL RESEARCH 2010
to PO and activates it for ring-opening. At the same time, CO2 is acti-
vated by the Lewis base centre to produce intermediate 1. The chlo-
ride ion act as a nucleophilic reagent, attacking the less-hindered
carbon atom of the coordinated PO to produce the requisite metal
alkoxide intermediate 2.30 When both the PO and CO2 have been
activated, PC is transformed by an intramolecular cyclic elimination
reaction. The bulky peripheral substituents in the tetrapyrazolyl por-
phyrins could block the binding sites of the metal, thereby inhibiting
the metal centres from activating and opening the epoxide at too high
or too low a temperature. This might be cited to explain why propyl-
ene carbonate was obtained at a relatively lower yield above 100 °C
or below 100 °C. In addition, the pyrazole ring with a lower alkalinity
might account for the lowered catalytic activity of [CoCl(TPPyP)] as
compared with DMAP at a reaction temperature of above 100 °C.
This mechanism can explain why [Co(TPPyP)] has no catalyst
activity even in presence of DMAP, because there is no chloride ion to
open the PO ring to generate intermediate 2. This mechanism might
also be cited to explain the gradual loss in activity of [CoCl(TPPyP)]
when the reaction temperature is above 100 °C, as the molar
equivalents of CO2 in solution are gradually diminished with the rise
of reaction temperature, and this would lead to reduced formation of
intermediate 3, thereby reducing the reaction rate.
Received 10 February 2010; accepted 19 May 2010
Paper 101001 doi: 10.3184/030823410X12762744367622
Published online: 28 July 2010
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can be used to prepare propylene carbonate from carbon diox-
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